Going from 100 lbs to 65 lbs is a pretty radical reduction. Unless the rough casting is way overdesigned there will be no easy way to do this.
Just about any "solution" will likely need to be equally radical and potentially economically unfeasible unless the valve is being used in some exotic...
I usually deal with steel and I have general understanding of NACE requirements for carbon and stainless steels. I have a requirement for ductile iron and I am a little unsure.
NACE states that ASTM A395 ferritic ductile iron is acceptable. However there are multiple grades under A395 (65-45-12...